Light waves propagating through an inhomogeneous space are subject to changes caused by interference, refraction, and diffraction. Thus, for example, light waves traveling along a path through a varying density gas and reflected by a mirror back along the same path to their source are likely very different than when initially radiated from the source. However, by replacing the mirror with a device that produces optical phase conjugate light waves, a different result is obtained. The optical phase conjugate light waves returning to the source along the same path are phase reversed, but otherwise are in the same state as the light waves originally emitted by the source. Thus, optical phase conjugation appears to reverse time by "undoing" the changes in the light waves caused by an inhomogeneous medium. An optical phase conjugated light wave therefore compensates for the inhomogeneities or distortion of the intervening space between the source and the device that produces the phase conjugate light waves.
One method of generating phase conjugate light waves employs stimulated Brillouin scattering. High intensity coherent light emitted by a laser is directed at a cell filled with a gas, liquid, or solid. The light causes periodic changes in the density of the material in the cell that also alter the index of refraction of the material in a corresponding periodic pattern. These periodic density fluctuations in the material scatter the light, reflecting a portion of it. The reflected light interferes with the incident wave, causing further density variations in the medium. The cumulative effect of this process continues, eventually creating a "reflected" optical phase conjugate light wave that emerges from the cell in the opposite direction from that traveled by the incident light emitted by the laser. However, the intensity threshold necessary to initiate Brillouin scattering typically requires a source having over a million watts/cm.sup.2 intensity. Another disadvantage of this method is that the resulting optical phase conjugate wave is of a slightly different frequency than the source light wave.
Fortunately, optical conjugate light waves can be produced by an alternative method that does not require as powerful a light source. This alternative method is called four-wave mixing because it involves the interference of four light waves inside a non-linear medium. In the prior art, all four light waves are of the same wavelength. One of the four light waves is referred to as a probe wave, i.e., a light wave for which an optical phase conjugate light wave is desired, with a frequency, .nu..sub.p. The optical phase conjugate light wave is the second of the four waves, and the other two light waves are called "pump waves". These two pump waves, which are of the same frequency, .nu..sub.1 and .nu..sub.2, are directed generally from opposite sides into the non-linear medium, which may comprise a dye coating on a glass plate. Interference between the probe and pump waves within the non-linear medium produces the optical phase conjugate light wave, and its frequency, .nu..sub.c, is equal to the sum of the two pump wave frequencies less the probe wave frequency, .nu..sub.c =.nu..sub. 1 +.nu..sub.2 -.nu..sub.p. The optical interaction in the non-linear medium uses energy from the two pump waves in producing the optical phase conjugate light wave, and the preceding equation is an expression of energy conservation in respect to this process. If the pump waves are of the same frequency, .nu..sub.0, but differ in frequency from the probe wave by an amount, .DELTA..nu., the phase conjugate wave frequency is simply: .nu..sub.c =.nu..sub.0 -.DELTA..nu..
Conservation of momentum is expressed by a similar relation between the wave vectors respectively associated with the four waves. Since the pump waves are at the same frequency, but counterpropagate in opposite directions and therefore cancel, the wave vector of the conjugate wave is k.sub.c =-k.sub.p, where k.sub.p is the wave vector of the probe wave, indicating that the phase conjugate wave is equal in magnitude, but travels in precisely the opposite direction of the probe wave.
The present invention is directed at a particular application of the optical phase conjugation phenomena--specifically, tracking the velocity and determining the position and range of one or more targets. Conventional Doppler laser radar ranging devices require a local oscillator and are limited in their ability to resolve the velocity and range of targets closing at extremely high velocities, particularly multiple targets spread over a relatively wide field of view. Such targets can present such large Doppler frequency shifts that their velocities and ranges cannot be accurately determined. In addition, a beam steering mechanism is often required to track the targets in order to keep the receiving device oriented properly to gather the light reflected from the target. Any distortion in the path between a conventional Doppler laser radar site and the target, for example, due to variations in air density, can seriously degrade the modulation efficiency of the received signal.
Accordingly, it is an object of the present invention to provide a velocimeter that can accurately determine the line-of-sight velocity of targets, including those that produce a high Doppler frequency shift in light reflected from the targets. Further, it is an object to resolve the position and range of targets in a wide field of view, even in the presence of intervening atmospheric distortion. A still further object is to provide a self-steering range/velocimeter that can track targets over a relatively wide field of view without use of a beam steering mechanism. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.